1. Field of the Invention
The present invention relates to a method of and an apparatus for producing single-crystalline diamond through vapor-phase synthesis, and more particularly, it relates to a method of and an apparatus for producing relatively large-sized single-crystalline diamond of at least about 10 mm by 10 mm, which is applied to a cutting tool, a precision tool, a semiconductor material, an electronic component, an optical component or the like.
2. Description of the Prior Art
Diamond, which has many excellent properties such as high hardness, high heat conductivity and transparency, is widely employed as material for various tools, optical components, semiconductors, electronic components and the like, and its importance is expected to increase in the years ahead.
While naturally produced diamond has been applied to industrial use in the past, artificially synthesized diamond is generally employed at present. In general, relatively large-sized polycrystalline diamond is artificially produced by vapor-phase synthesis such as plasma CVD (chemical vapor deposition) (refer to "Applied Physics", Vol. 55, No. 7, 1986, pp. 640 to 563, for example). In such polycrystalline diamond, however, a polycrystalline diamond layer which is formed on a substrate is so heterogeneous that a sufficiently flat surface cannot be obtained by polishing. In application to a superprecision tool, an optical component or a semiconductor particularly requiring a flat surface, therefore, it is necessary to employ single-crystalline diamond having homogeneous crystal orientations. While such single-crystalline diamond is naturally produced, the output thereof is so small that a method of artificially producing single-crystalline diamond has been studied in general.
With the vapor-phase synthesis technique of plasma vapor deposition, single-crystalline diamond is generally produced by vapor-depositing single-crystalline diamond on a base material with a low- or high-temperature plasma which is generated by mixing at least one carbon source selected from hydrocarbon, carbon oxide, alcohol and acetone with hydrogen, oxygen, water, nitrogen or halogen for preparing source gas, and then decomposing and activating the source gas with a direct current, a low-frequency alternating current, a high frequency or a microwave. Alternatively, in the thermal filament method, single-crystalline diamond is produced by vapor-depositing single-crystalline diamond on a base material material with a thermoionic emissive material such as a tungsten filament that is heated to a high temperature. Using either method, the single-crystalline diamond can be homoepitaxially grown on a single-crystalline diamond base material or heteroepitaxially grown on a non-diamond base material, depending on the type of the employed base material. In the case of homoepitaxially growing the single-crystalline diamond, the base material may be prepared by bonding a plurality of single-crystalline diamond base materials that are adjacently arranged in the same plane with each other to attain a large area for mosaic growth (refer to Japanese Patent Laying-Open No. 3-75298 (1991), Diamond and Related Material, No. 4 (1995) pp. 1025 to 1031 etc.).
In the conventional homoepitaxial growth, however, abnormal growth is caused on side surfaces due to growth of twins or secondary nuclei, while transverse spreading is inhibited by polycrystalline diamond growing from a base material holder. In relation to the heteroepitaxial growth, on the other hand, a problem of inconsistency in in-plane orientation has not yet been solved although a technique for attaining regular orientations on the upper surface has been developed. With mosaic growth, inconsistent junctions between the single-crystalline diamond base materials remain as interfaces to cause abnormal growth, and hence the area of the single-crystalline diamond cannot be increased. In order to prevent formation of the interfaces, the angles of the base materials must be completely regularized.
In general, further, the base material holder is prepared from a material such as silicon, silicon carbide or cubic boron nitride that readily forms a compound by reaction with carbon, disadvantageously leading to formation of polycrystalline diamond that inhibits growth of single-crystalline diamond in the vicinity of the base material.